Photosystem II (PSII) is a membrane protein complex that uses light to catalyze water oxidation and quinone reduction. The oxygen-evolving complex (OEC) of PSII is the source of the atmospheric oxygen that we breathe. Because atmospheric oxygen is essential to sustain all aerobic life on Earth, the function of the OEC is of major importance to human health. The proposed studies will provide new information on how oxygen is produced by the OEC. The recent 3.5 [unreadable] crystal structure of PSII provides the first structural model of the OEC. The revealed geometry of the OEC fits well with the water-splitting mechanism advanced by this group in which, a nucleophilic Ca-bound water attacks the oxygen of a Mn(V)=O species in the O?O bond- forming step. Following from the new structure, the OEC will be characterized by using a combination of biophysical and computational studies of PSII and bioinorganic Mn model chemistry. The long-term objectives are to develop an understanding at the molecular level of the structure and assembly of the OEC, the mechanism of water oxidation, the electron-transfer properties of PSII and the damaging side reactions that occur in the process of water oxidation. This project is divided into two specific aims: (1) to characterize the structure and function of the OEC, and (2) to use Mn coordination complexes to aid in the interpretation of biophysical studies of the OEC. In aim 1, studies are proposed to characterize the function of Ca in the OEC by using cation- and/or an ion-substituted PSII, to test the proposal that CP43-Arg357 functions as a catalytic base in the O-O bond-forming step by study of CP43-Arg357 mutants, and to use computational modeling to gain insight into the structure and function of the OEC. In aim 2, Mn coordination complexes will be used to gain insight into how the substrate waters are bound in the OEC by measuring the exchange of 18-O-labeled water with water/hydroxide/mu-oxo species in a series of Mn coordination complexes that model the structural properties of the OEC. In addition to providing a clearer picture of the water-oxidation chemistry of PSII, the proposed studies will provide insight into the mechanisms of other metalloenzymes that are essential to human health such as those that use oxygen or hydrogen peroxide as a substrate.